This invention relates to wave propagation devices and more particularly to a filter adapted for use with a wave propagation structure to filter out harmonic components of a wave propagating along the wave propagation structure.
Wave propagation structures, particularly slow-wave structures, have been used extensively is travelling wave tubes (TWT) to provide controlled reaction of a travelling electromagnetic wave with high velocity electrons in an electron beam. A helix is frequently employed as a slow-wave structure in a TWT designed for wide-bandwidth operation, such bandwidths being in excess of one octave. Unfortunately, such wide-band TWT's have, in the past, been restricted to relatively low power outputs as compared to narrow-band TWT's employing resonant slow-wave structures such as coupled cavity resonators.
In the operation of a TWT utilizing a helix as the slow-wave structure, a beam of electrons is transmitted down the helical axis within the region enclosed by the helix, generally known as the interaction region. An input RF signal is coupled to the helix and travels along the helix in the form of a slow wave having an electromagnetic field both within and without the helix. The pitch of the helix is selected so that the velocity of the slow wave along the helical axis is approximately equal to the velocity of the electrons in the interaction region so that these electrons can interact with the slow wave to impart energy to and amplify the slow wave. For operation of a TWT at relatively high values of output power, relatively high values of electron beam current and voltage are utilized. Since an increased beam voltage results in an increased electron velocity, the pitch of the helix is relatively large to provide a higher slow-wave velocity.
A problem arises in the operation of a wideband TWT at high power levels because the increased helical pitch affects the interaction of the electron beam with the slow wave travelling along the helix. The electric field distributions of the various modes of the slow wave, particularly the forward wave and the backward wave, in the interaction region vary in accordance with the pitch of the helix. While both tightly wound and stretched out helices provide good interaction between the forward wave and the electron beam, a stretched out helix provides significantly greater interaction between the backward wave and the electron beam than does a tightly wound helix. As is well known, the backward wave transports energy in the direction reverse to the forward wave and is amplified by the electron beam, the amount of amplification depending on the extent of the interaction between the backward wave and the electron beam. When sufficient interaction is obtained, the TWT oscillates with the result that the output signal of the TWT bears little or no resemblance to the input signal. Such interaction with the attendant oscillation has proved to be an upper limit to power output in travelling wave tube amplifiers of the prior art employing a helical wide-band slow-wave structure.
It is, therefore, an object of the present invention to increase the power of a wideband travelling wave tube.
It is also an object of the present invention to provide a means for filtering out a backward wave propagating along a slow-wave structure.
It is furthermore an object of the present invention to improve the stability of a travelling wave tube amplifier employing a helical slow-wave structure wherein the helical pitch has been increased to such a high value that the helix advances along its axis a distance of almost one helix diameter during a single turn of the helix.